No, the digitals don't use a rack. They don't need one because they don't have a spring loaded anti-backlash gear keeping control of a dial needle. The digitals just have a surface roughened by abrasive blasting (or so it appears) that I think they roll a little encoder over. The electronics counts encoder increments rolling by from whatever starting place it happens to be at. I think they are capacitive rather than optical in those low powder devices.

The main advantage to the digitals, other than display visibility, is you can zero them anywhere along the way to get difference readings to compare two objects. The main disadvantage is that a fellow skilled enough to do it can't read between the least significant digits the way he can the graduations of a dial caliper. Sometimes useful. Also, the dial calipers never have their batteries run down. But overall, the digitals are handy not only because of that zeroing feature, but also because they will switch the display to millimeters at the push of a button so you don't have to own a separate metric version for decoding CIP drawings or foreign military specs.

A digital caliper perfectly adequate for reloading is currently on sale at Harbor Freight for $11.99. If you want to measure slugs of bores and chamber throats, I recommend you get an OD thimble micrometer for its higher resolution. Again, Harbor Freight's $21 unit is adequate for the purpose if you can read a Vernier scale. If not, they have a digital with half ten-thousandths resolution for $35. I recommend their micrometer stand to help get the most accurate readings, and that's on sale also, for $7.